Delete draft changes and patchsets

Adds ability to delete draft changes and patchsets that are not meant
or fit for code review. Deleting a draft patchset also deletes the
corresponding ref from the repository and decrements the next patch
set number for the change if necessary. Deleting a draft change
deletes all of its (draft) patchsets.

Change-Id: I04abeb67b64dd2366514e74d23f83066d409904e
19 files changed